Ozone has a range of uses similar to that of chlorine, including the bleaching of clays and pulp and the treatment of drinking water swimming pool water, municipal secondary effluents, high quality water (eg for electronic component manufacturing) and odours. Because chlorine produces chlorinated organics, which present long term toxicity hazards at low concentrations in water, there is a large potential market for a less toxic replacement for chlorine. Ozone has the potential to take over from chlorine. In addition, ozone may be used in organic synthesis for the oxidation of the carbon-carbon double bond, medical therapy and serialization. Examples of chemical syntheses using ozone include the production of silver oxide, heliotropin, pelargonic acid, azelaic acid, peracetic acid, germicides, steroids, Nylon-9 precursor and the separation of cerium from other rare earths.
Ozone has a short half-life, and because of its highly reactive nature, tends to decompose in contact with most metals and oxides and is potentially explosive when concentrated either as a gas or liquid or when dissolved into solvents or absorbed into gels. Transportation therefore is not practical, and consequently it is desirable to generate ozone at the point of use. Currently, the major method of ozone generation adopted in industry and the utilities is corona discharge. In this process an alternating, very high voltage is discharged through a dry air or oxygen steam, producing a flow of gases containing up to approximately 2% ozone where an air stream is used, and up to approximately 4% ozone where oxygen is used. Corona discharge methods for ozone production have several disadvantages. In particular, the equipment required is capital intensive and bulky. In addition, the process requires the feed gas to be cleaned, compressed, cooled and dried before passing to the discharge tube. The use of oxygen rather than air where higher yields of ozone are required, adds significantly to the cost. The low ozone:oxygen ratios mean that dissolution of the ozone into water or other process medium is difficult, with low rates of transfer. An additional drawback is that appreciable amounts of nitrogen oxides are produced when an air feed is used, ultimately producing nitric acid if the output stream contacts water.
Another method of ozone generation is the electrolysis of water. In this process, water is electrolysed between an anode and a cathode. At the anode, oxygen and ozone are evolved as a mixture. The generation of ozone is most efficient at low temperatures. At the cathode, hydrogen may be evolved, or oxygen may be reduced at a gas porous air or oxygen depolarised cathode, reducing the overall voltage required to drive current through the cell.
Gas porous electrode technology is highly developed, and the general principals of gas porous electrode fabrication are well known to those skilled in the art [See Fuel Cell Handbook, A J. Appleby and F R Foulkes, Pub. Van Nostrand Reinhold, 1989: and references therein, EP 0357077A3, U.S. Pat. No. 4,564,427, U.S. Pat. No. 4,927,514]. In general terms, a gas porous electrode is an electronically conducting structure including a high-surface area body and a suitable catalyst. For example a catalyst layer and a hydrophobic support layer may be deposited on an electronically conducting substrate. Air or oxygen breathing cathodes are generally constructed so as to produce the maximum area of a three-phase interface, that is the maximum area of interface between the solid catalyst material, the electrolyte medium, and the gas phase (which in the case of ozone generation, is air or oxygen). This may be achieved by the admixture of the catalyst with a wet-proofing agent such as polytetrafluoroethylene (ptfe), or fluorinated ethylene propylene (fep), or a wax, etc. The partially wet-proofed catalyst layer, which is gas permeable, may then be deposited in a uniform thin layer over a hydrophobic support material which should be gas permeable but prevent the passage of electrolyte. This support material may take the form of form example porous polytetrafluoroethylene film, wet-proofed carbon fiber paper, or mixtures of carbon with polytetrafluoroethylene or similar hydrophobic binders. A suitable conducting substrate such as carbon paper or cloth or a carbon fluoro-polymer mixture may be incorporated into the electrode, to enable uniform distribution of current across the electrode. Said conducting substrate may typically take the form of a woven mesh, a felt of randomly-orientated fibers, or a pierced sheet which may also have been stretched to form an expanded metal mesh. The material of the mesh is not critical, as long as it has good conductivity, and resistance to corrosion by the electrolyte. For example, it may be stainless steel, monel, Hastelloy, silver, copper, nickel, carbon or one of these covered by a more corrosion-resistant coating such as platinum or gold.
For effective diffusion of gases to the catalyst layer, and ions to the electrolyte, electrodes have previously been fabricated as this as possible to minimum diffusion losses.
In the electrolytic generation of ozone, the oxygen source may be pure oxygen or may be supplied from the atmosphere. The oxygen reduction process at the cathode is much improved by operation at higher temperatures. Therefore there is direct conflict in the electrochemical generation of ozone, between the requirements for operation of the anode, and the requirements for operation of the cathode. To avoid excessive ohmic resistance losses, the gap between anode and cathode must be kept small. This presents a number of problems. For example, operation of the anode at very low temperatures such as chilling the back face to below -40.degree. C., can be conductive cooling through the electrolyte reduce the temperature of the cathode structure to below 0.degree. C. This can lead to condensation of moisture from the feed air supply. Particularly at low current density, the coldness of the anode and electrolyte can so chill the air cathode that this moisture can form a layer of frost on the air face of the electrode, thus reducing the ingress of oxygen by diffusion, and limiting the cathode performance. This imposes a need to reduce the humidity in the air supply to very low levels.
Further, the absorbed moisture may migrate through the cathode to dissolve in the electrolyte, thereby producing a commensurate increase in overall volume and a reduction in electrolyte concentration. In extreme cases this may lead to a significant degradation of cell efficiency. In addition, the cooling requirement for the anode is increased by the thermal input via both the air passing over the cathode and the heating effect of ohmic resistance losses within the cathode.
Conventionally, in such a system the air supplied to the cathode is dried by means of a desiccant scrubber, such as silica gel, a pressure swing absorption drier, or a low temperature condenser, in order to reduce the dew point well below that likely to be observed at the air face of the cathode. Extra electrical power, and/or materials for the driers, are required.